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Community and Q&A

Using Forced-Air Ducts for ERV

rohman | Posted in Mechanicals on

I am looking to install a ERV in a home that has two furnaces. One furnace serves the basement and 1st floor (furnace #1) and the other serves the second floor (Furnace #2).

Square footages are approximately:
1500 first floor
2000 second floor
1500 basement

Climate zone 5 (Chicago)

For cost reasons I would like to install only one ERV system for the entire property. Also for cost reasons I would like to utilize the forced air system(s) duct work.

What does the community think about hooking up the ERV as follows:

Return air (to ERV) hooked up to return air plenum of furnace #2
Fresh air (From ERV) hooked up to return air plenum of furnace #1

Ideally their would be a reverse interlock with both furnaces so the ERV only runs when when both furnaces were off. During high demand periods (coldest or warmest days) this may be very infrequent, and not practical.



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  1. GBA Editor
    Martin Holladay | | #1

    I advise against your plan.

    I'm a strong believer in dedicated ventilation ductwork for any HRV or ERV. Using forced-air ductwork designed for a heating or cooling system is always a compromise, and any time you end up considering how to move ventilation air when a furnace fan (or air handler fan) is involved, you get big problems -- either you end up using the furnace fan, which is an energy hog that unbalances the system, or you end up posting the kind of question you just posted -- a question with no good answer.

    Ventilation ductwork is relatively small diameter -- usually 4 inches. If necessary, box it it.

    More information here: Ducting HRVs and ERVs.

  2. rohman | | #2


    Assuming dedicated duct work is not an option. Is it your opinion that setting up the ventilation in the manner I described above is better, worse, or the same as a typical erv setup using the duct work from a single furnace air handler?

    In my mind the above option would be superior to using the air handler from a single furnace. With the above you would not have the airflow short circuit issues you run into with a single furnace distribution option, also the return and supply from/to the erv would be from seperate areas if the home.

  3. GBA Editor
    Martin Holladay | | #3

    The main hurdle to your plan is the need to install a "reverse interlock with both furnaces so the ERV only runs when when both furnaces were off."

    Good luck finding an electrician who can do that. (Of course, maybe you plan to install the logic and relays yourself.)

    And good luck diagnosing and fixing future control problems when a relay fails in a few years. HVAC contractors who arrive on the scene will simply scratch their heads.

    And, as you point out, when your furnaces are operating, you won't get any ventilation.

    Maybe you should just install a bath fan or two, or a pair of Lunos fans.

  4. rohman | | #4


    Thank you for your help.

    I figured I would have to install the relay logic myself, which isn't a problem. I'm comfortable with the "sparky" stuff, that is the easy part. My concern is, as you reiterated, what happens when there is a need for ventilation and one or both furnaces are operating.

    It sounds like I need to find an expert expert in duct design to answer that question.

    Originally the plan was to go with a cheap simple solution along the lines of your suggestion to of install a couple of bath fans. The plan was to install supply only ventilation via a fresh air intake. Problem there is I could not figure out how to stop condensation from forming on the intake duct, and I thought a controlled mixing of the airstreams within an ERV would solve that.problem. Maybe I am mistaken with that thought too?

  5. Expert Member
    Dana Dorsett | | #5

    As a retrofit where dedicated ducts can't be installed it's often cheaper/easier to go with the ceramic core reverse-cycling approach such as Lunos or Twinfresh Comfo, etc..

    The amount of power used running HVAC air handlers merely for ventilation air is ridiculous. Actively ventilating the most important or largest spaces with a ductless ceramic core HRV and duty-cycling the air handlers at some very low duty cycle during periods of low or zero heating/cooling load would be enough for most houses.

  6. jacobw | | #6

    If the HVAC system consists of a two-stage handler that will run continuously, does that alleviate concerns about sharing ductwork and using the air handler to distribute the ventilated air?

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